Semiconductor Device and Fabrication Method Thereof

ABSTRACT

For forming a gate electrode, a conductive film with low resistance including Al or a material containing Al as its main component and a conductive film with low contact resistance for preventing diffusion of Al into a semiconductor layer are laminated, and the gate electrode is fabricated by using an apparatus which is capable of performing etching treatment at high speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/743,189, filed May 2, 2007, now allowed, which is a divisional ofU.S. application Ser. No. 10/105,282, filed Mar. 26, 2002, now U.S. Pat.No. 7,238,600, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2001-091275 on Mar. 27, 2001,all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices including a drivecircuit as formed using a semiconductor element with a semiconductorfilm as an active layer (a semiconductor layer including a channelformation region, a source region and a drain region) and also to amanufacturing method of the same. Note that typical examples of thesemiconductor element are transistors including, although not limitedto, field effect transistors such as metal oxide semiconductor (MOS)transistors and thin-film transistors (TFTs). In addition, the presentinvention relates in particular to large size display devices (more than20 inches) wherein resistance of wiring lead becomes a problem insupplying signal, and also to a fabrication method thereof.

2. Description of the Related Art

To realize reduction in weight and power consumption of portableinformation equipment such as a mobile telephone and a notebook PC,small-size or middle-size liquid crystal display devices have generallybeen used as display units thereof due to merits of such liquid crystaldisplay devices.

Further, it is becoming a more active trend to enlarge the market of theliquid crystal display device up to a television (TV) and also to designa TV at home by using the liquid crystal display device in place of atraditional cathode ray tube (CRT). However, it is necessary tosimultaneously satisfy higher fidelity and higher brightness in additionto the enlargement to realize a liquid crystal TV.

Further, with an increase in size of a display device, the number andlength of wirings and resistivities of the wirings increase accordingly.An increase in wiring resistance would cause a delay in transmittingsignals toward lead terminate ends to in turn badly affect the resultantdisplays. Therefore, a technique for reducing the wiring resistancebecomes inevitable in order to widely spread the liquid crystal displaydevices to ordinary homes.

So far, as the technique for reducing wiring resistance, a method ofwidening line widths of the wirings and increasing the film thickness ofthe wirings have been considered, and actually, it has been realized toreducing the wiring resistance. However, the former is encountered withan enlargement of element areas in a pixel portion due to widening ofthe lead widths to result in decreasing an aperture ratio and to make itimpossible to obtain higher brightness required. Alternatively, with thelatter method, the increase in film thickness of wirings results in anincrease in difference between a layer formed under the wirings and alayer formed on the wirings to cause production yields to decrease dueto problems such as disconnection as a result of reduction of coatingproperties in fabricating insulating films and metal films forelectrodes.

Additionally, although there is a method of using aluminum (Al) orcopper (Cu) as materials with low resistance for wirings, which sufferfrom defects of less corrosion resistivity and less thermal resistivity.It occurs as problems that projections such as hillocks and whiskers orthe like are formed by thermal treatment, and that aluminum atoms behaveto diffuse into a channel formation region to result in operationdefects of TFTs or degradation in TFT characteristics. In this way,while it is not easy to form a suitable gate electrode of a TFT by usingthe above-noted metal materials, no materials are less in resistancethan aluminum (Al) or copper (Cu). The above serves as a problem againstproducing a liquid crystal display device with a large screen.

In this way, the above-stated problem becomes more appreciable withincreasing performance required for a semiconductor device which has aplurality of integrated circuits such as an active matrix type liquidcrystal display devices.

SUMMARY OF THE INVENTION

The present invention is performed in view of the problem stated above,and the object is to provide, in a semiconductor device typicallyrepresented by an active-matrix type liquid crystal display device whichhas a circuit using semiconductor elements, a technique for realizinglow resistivity of wirings required for enlargement and higher precisionwithout increasing the number of steps in the manufacturing process, andfurther provide a method for fabricating a semiconductor device at lowtemperatures (temperatures lower than or equal to the distortion pointof a glass substrate) to use glass substrates with a low cost.

The invention has the following structure. A conductive film containingW as its main component is used for a first layer to form a gateelectrode in order to prevent aluminum of the gate electrode fromtranspiring and diffusing into a channel formation region with a filmwhich contains Al as its main component and is low in resistance as asecond layer and a film which contains Ti as its main component, tofabricate the gate electrode with a laminate structure of the abovematerials by using an apparatus which is capable of performing etchingtreatment at high speeds.

Additionally it is required that a TFT disposed in each of a variety oftypes of circuits be fabricated as a TFT in accordance with the functionof the circuit. For example, it is desirable that the TFT provided in adriver circuit required to achieve high-speed operations be designed tohave a specific structure with great emphasis on an increase inoperation speed and at the same time on suppression of any possibledegradation as an appreciable problem due to hot carrier injection. Itis known as such structure that an LDD region as provided between achannel formation region and a drain region has a concentration in orderthat the concentration of an impurity element gradually gets highertoward the drain region. With the structure, the effect of relaxingelectric field becomes more remarkable in a depletion layer adjacent tothe drain region.

In order to form the LDD region with the above-noted concentrationgradient of the impurity element, the present invention use a method ofaccelerating by electric fields an ionized impurity element for givingone conductivity type to pass through a gate insulating film and thenadd into a semiconductor layer. In addition, with the present invention,a gate electrode with a tapered shape is formed to have a thicknessgradually increasing from an end portion toward inside by etching, andit is considered that the impurity element is somewhat added to thesemiconductor layer through the tapered shape. In the present invention,the LDD region is formed in order that the concentration of the impurityelement changes in a direction along the channel length of a TFT,without increasing the number of steps (without increasing the number ofmasks).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one preferred embodiment of the invention:

FIG. 2 is an SEM photograph showing an observation result of a laminatedconductive film as etched:

FIG. 3 is a diagram showing one example for reduction to practice of theinvention:

FIG. 4 is a diagram showing one example for implementation of thisinvention:

FIG. 5 is a diagram showing one example for implementation of theinvention:

FIG. 6 is a diagram showing one example for implementation of theinvention;

FIG. 7 is a diagram showing one example for implementation of theinvention:

FIG. 8 is a diagram showing one example for implementation of theinvention:

FIG. 9 is a diagram showing one example for implementation of theinvention:

FIG. 10 is a diagram showing one example for implementation of theinvention:

FIG. 11 is a diagram showing one example for implementation of theinvention:

FIG. 12 is a diagram showing an example of electrical equipment:

FIG. 13 is a diagram showing an example of electrical equipment; and

FIG. 14 is a diagram showing an example of electrical equipment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiment mode, it is described that a method of fabricating adisplay device that is adaptable for the enlargement and high imagequality by using materials with low resistance without increasing thenumber of steps, with reference to FIGS. 1 and 2.

A base insulating film 1002 and a semiconductor layer 1003 are formed ona substrate 1001, and a gate insulating film 1004 with a film thicknessof 40 to 150 nm is formed by plasma CVD method, sputtering method orlow-pressure CVD method or the like. Then, On the gate insulating film1004 a three-layered conductive film formed of a first conductive film1005, a second conductive film 1006, and a third conductive film 1007 isformed, thus, a mask 1008 is formed (FIG. 1A).

The present invention employs, as a conductive film for a gateelectrode, a laminate structure of a conductive film with low resistancesuch as Al, Cu, or one of a chemical compound material and alloymaterial containing an element selected from the group consisting of Aland Cu as its main component: a conductive film with excellent heatresistance such as W, Mo, Ta, or one of a chemical compound material andalloy material containing an element selected from the group consistingof W, Mo, and Ta as its main component; and a conductive film with lowcontact resistance such Ti, or one of a chemical compound material andalloy material containing Ti as its main component. The laminatestructure of these conductive films are etched repeatedly to therebyform a gate electrode without increasing the number of masks and furtherto form an impurity region in a semiconductor layer to obtain a TFT withthe required performance.

In etching the conductive films, the etching rate of the conductive filmto be processed, the selective ratio of an insulating film for a basefilm to the conductive film, and so forth should be considered. If theselective ratio is small, processing becomes difficult to make itdifficult to form a TFT with a desired shape becoming.

Then, in order to obtain the optimal processing method, the experimentof etching conditions was performed after preparing a sample that asequential laminated structure on a quartz substrate of an insulatingfilm of the same material as a gate insulating film, a tungsten filmwith a film thickness of 50 nm, an alloy film of aluminum and titanium(Al—Ti) with a film thickness of 500 nm, and a titanium film with a filmthickness of 30 nm in the Embodiment Mode. Suppose that the tungstenfilm is the first conductive film 1005, the alloy film of aluminum andtitanium (Al—Ti) is the second conductive film 1006, and the titaniumfilm is the third conductive film 1007, for the purpose of convenience.

First, BCl₃, Cl₂ and O₂ are used as etching gas and a gas flow rateratio thereof is set at 65/10/5 sccm, and RF (13.56 MHz) electricalpower of 450 W is supplied to a coiled electrode at a pressure of 1.2 Pato thereby produce a plasma and to perform etching. RF (13.56 MHz)electric power of 300 W is supplied also to the substrate side (samplestage) to apply a substantially negative self-bias voltage for theetching. Subsequently, CF₄, Cl₂ and O₂ are used as etching gas and a gasflow rate ratio thereof is set at 25/25/10 sccm, and RF (13.56 MHz)electrical power of 500 W is supplied to a coiled electrode at apressure of 1.0 Pa to thereby produce a plasma and to perform etching.RF (13.56 MHz) electric power of 20 W is supplied also to the substrateside (sample stage) to apply a substantially negative self-bias voltagefor the etching. A photograph observed by SEM immediately after havingdone the etching under the above conditions is FIG. 2. With this etchingtreatment, a first electrode 1009 a, a second electrode 1009 b and athird electrode 1009 c are formed from the first conductive film, thesecond conductive film and the third conductive film. The laminatedconductive film shown in FIG. 2 is regarded as a gate electrode 1009with the first shape, which consists of the first electrode 1009 a,second electrode 1009 b and third electrode 1009 c.

Then, FIG. 1B shows a schematic diagram of a state in which an impurityelement that gives one conductivity type is doped in a self-alignmentfashion using as a mask the first shaped gate electrode 1009 etched asshown in FIG. 2.

The first shaped gate electrode 1009 has a tapered portion at its edge,and the gate insulating film also have a portion etched from its surfaceto a certain degree. The impurity element for giving on conductivitytype is doped through the gate insulating film into the semiconductorlayer formed thereunder. In addition, it is also possible to dope theimpurity element somewhat through the edge portion of the first shapedgate electrode with the taper shape and a nearby portion of the edgeportion into the semiconductor layer formed thereunder. Thus, animpurity region (A) 1010 with the doped impurity at a high concentrationis formed. At this time, it is considered that there is the possibilitythat the impurity element is doped into the semiconductor layer throughthe tapered portion of the first electrode 1009 a and the gateinsulating film to form an impurity region which overlaps the firstshaped gate electrode.

Then, BCl₃, Cl₂ and O₂ are used as etching gas and a gas flow rate ratiothereof is set at 65/10/5 sccm, and RF (13.56 MHz) electrical power of450 W is supplied to a coiled electrode at a pressure of 1.2 Pa tothereby produce a plasma and to perform etching. RF (13.56 MHz) electricpower of 300 W is supplied also to the substrate side (sample stage) toapply a substantially negative self-bias voltage for the etching.Subsequently, CF₄, Cl₂ and O₂ are used as etching gas and a gas flowrate ratio thereof is set at 25/25/10 sccm, and RF (13.56 MHz)electrical power of 500 W is supplied to a coiled electrode at apressure of 1.0 Pa to thereby produce a plasma and to perform etching.RF (13.56 MHz) electric power of 20 W is supplied also to the substrateside (sample stage) to apply a substantially negative self-bias voltagefor the etching. With the etching treatment, a fourth electrode 1011 a,a fifth electrode 1011 b and a sixth electrode 1011 c are formed fromthe first electrode 1009 a, the second electrode 1009 b and the thirdelectrode 1009 c. The laminated structure consisting of the fourthelectrode 1011 a, fifth electrode 1011 b and sixth electrode 1011 c isregarded as a second shaped gate electrode 1011.

Then, a schematic diagram of a state, in which an impurity element forgiving one conductivity type is doped in a self-align fashion with thesecond shaped gate electrode as a mask, is shown in FIG. 1C.

In the second doping processing, the impurity element for giving oneconductivity is doped, and an impurity region (B) 1012 is formed. Notethat although the impurity element is added into the impurity region (A)1010 formed by the first doping processing, the influence is negligiblesince the concentration in the second doping processing is low. Althoughthe newly formed impurity region (B) 1012 is formed with the fourthelectrode 1011 a, fifth electrode 1011 b and sixth electrode 1011 c usedas a mask, it is considered at this time that there is the possibilitythat an impurity element is doped into the semiconductor layer throughthe tapered portion of the fourth electrode 1011 a and the gateinsulating film to form of an impurity region which overlaps the secondshaped gate electrode.

Thereafter, an interlayer insulating film 1013 is formed to cover thegate electrode 1011. Then, in the interlayer insulating film 1013, acontact hole that reaches a region 1010 of the semiconductor to becomeeither a source region or a drain region. Next, a wiring lead 1014 usedfor electrical connection of each TFT is formed.

As stated above, with the etching method of employing gas plasma in areduced pressure atmosphere with three layers of conductive filmslaminated, it becomes possible to form a gate electrode with a desiredshape by changing etching conditions. Further, by doping an impurityelement through the tapered portion of the gate electrode, it ispossible to form in the semiconductor layer a region in which theconcentration of the impurity element changes gradually.

Additionally, with respect to the inductively coupled plasma (ICP)etching method used to form the gate electrode of the present inventionwith conductive films with low resistance laminated, it is easy tocontrol plasma, and thus the method is applicable also for a substratewith a large-area to be processed.

Embodiment 1

In Embodiment 1, a method for simultaneously fabricating on the samesubstrate both a pixel portion and TFTs (p-channel type TFT andn-channel type TFT) of a driver circuit to be provided at the peripheryof the pixel portion is explained in detail with reference to FIGS. 3 to6.

In FIG. 3A and FIG. 4A, it is possible to use barium borosilicate glass,alumino-borosilicate glass, quartz or other similar suitable materialsas a substrate 100 although there are no particular limitations to thematerial thereof. On a surface of the substrate 100, an inorganicinsulating film is formed as a base insulating film 101 to have athickness of from 10 to 200 nm. A preferable example of the baseinsulating film is a silicon oxynitride film which is fabricated byplasma CVD method, and a first silicon oxynitride film 101 a made fromSiH₄, NH₃, N₂O is formed to have a thickness of 50 nm and then a secondsilicon oxynitride film 101 b made from SiH₄ and N₂O is formed to have athickness of 100 nm. The base insulating film 101 is provided to preventalkali metals contained in the glass substrate from diffusing into asemiconductor film to be formed later. In the case of using quartz asthe substrate, it will possibly be omitted.

As an amorphous silicon film 102 formed on the base insulating film 101,a semiconductor material containing therein silicon as a main componentis used. A typical example is either an amorphous silicon film or anamorphous silicon germanium film or the like, which is formed to have athickness of 10 to 100 nm by plasma CVD method, low-pressure CVD methodor sputter method. In order to obtain good crystals, it is recommendablethat the impurity concentration of oxygen and nitride or the likecontained in the amorphous silicon film 102 be reduced to a level lessthan or equal to 5×10¹⁸/cm³, and preferably 1×10¹⁸/cm³ or less. Further,if the concentration of oxygen within the amorphous silicon film ishigh, it will be difficult to release catalytic elements (especially,nickel) used during a crystallization process. Therefore, it isimportant in order to obtain a good crystalline semiconductor film thatthe oxygen concentration within the amorphous silicon film 102 is setless than or equal to 5×10¹⁸/cm³, and preferably 1×10¹⁸/cm³ or below.These impurities become a factor for inhibiting crystallization ofamorphous semiconductor materials, and also a factor for increasing thedensity of a trap center and a recombination center even aftercrystallization. Accordingly, it is desirable to employ CVD equipmentwith adaptability for ultrahigh vacuum which has mirror-surfacetreatment (electrolytic polishing treatment) within a reaction chamberand an oil-free vacuum evacuation system, in addition to the usinghigh-purity material gas.

The amorphous silicon film 102 thus formed is crystallized to therebyform a crystalline semiconductor film. As a method for suchcrystallization, the conventional laser annealing method, thermalannealing method or RTA method are employable.

It is preferable that, prior to the crystallization processing, thathydrogen contained in the semiconductor film be released away, and it isrecommendable that the crystallization be conducted after performingthermal treatment at 400 to 500° C. for about one hour to set an amountof the contained hydrogen at a level less than or equal to 5% of thenumber of all atoms contained in the semiconductor film, since it ispossible to prevent roughness of the surface. Generally, theconcentration contained hydrogen in an amorphous semiconductor film bysputter method or LPCVD method is lower than that of the amorphoussilicon film formed by plasma CVD methods. Additionally, it is knownthat even if an amorphous semiconductor film is formed by plasma CVDmethod, the concentration of contained hydrogen is formed in forming ata temperature of 400° C. or higher.

In Embodiment 1, a laser annealing method is used to performcrystallization of the amorphous silicon film 102. The lasercrystallization method can employ an excimer laser, YAG laser, YVO₄laser or the like, which is the pulse oscillation type or alternativelythe continuous emission type. In this case, the efficiency is good whenlaser light emitted from a laser oscillator is corrected and focusedinto a linear shape by an optical system to irradiate onto asemiconductor film. While the conditions for crystallization areselected appropriately, pulse oscillation frequency is set at 300 Hz anda laser energy density is set at 100 to 800 mJ/cm² (typically, at 200 to700 mJ/cm²) in the case of using an excimer laser. Alternatively, in thecase of using a YAG laser, it is preferable to use the second higherharmonic wave while the pulse oscillation frequency is set at 1 to 300Hz with the laser energy density being set at 300 to 1,000 mJ/cm²(typically 350 to 800 mJ/cm²). It may be performed to irradiate linearlycorrected laser light with a width of 100 to 1000 μm, e.g., 400 μm, overthe entire surface of the substrate while the overlap ratio of linearbeams at this time is set at 80 to 98%.

In addition, the laser crystallization method can be performed in theatmosphere, an atmosphere of an inert gas such as nitrogen, a reducedatmosphere or the like.

Subsequently, in order to form a semiconductor layer which includes achannel formation region, a source region, and a drain region, thecrystalline silicon film is etched to form semiconductor layers 103 to106. An impurity element which gives p-type may be doped to control thethreshold value (Vth) of an n-channel type TFT. Known examples of theimpurity element that gives p-type to semiconductor are XIII groupelements in the periodic table, such as boron (B), aluminum (Al),gallium (Ga) and the like.

Then, a gate insulating film 107 is formed to cover the semiconductorlayers 103 to 06 thus separated (FIG. 3B, FIG. 4B). The gate insulatingfilm 107 is formed by plasma CVD method or sputter method, and is formedof a silicon-containing insulating film to have a thickness of 40 to 150nm. The silicon-containing insulating film may be used as a single layeror to be a laminate structure.

On the gate insulating film 107, a first conductive film 108 with a filmthickness of 20 to 100 nm, a second conductive film 109 with a filmthickness of 100 to 400 nm, and a third conductive film 110 with athickness of 20 to 100 nm (FIG. 3C, FIG. 4C) are formed. Although atungsten film with a film thickness of 50 nm, an alloy film of aluminumand titanium (Al—Ti) with a thickness of 500 nm, and a titanium filmwith a film thickness of 30 nm are sequentially laminated on the gateinsulating film 107, the first conductive film 108, the secondconductive film 109, and the third conductive film 110 are not limitedonly to these materials.

Next, as shown in FIG. 3D and FIG. 4D, a resist mask 111 is formed byexposure process, followed by executing first etching treatment forforming gate electrodes and wirings. It is preferable to use aninductively coupled plasma (ICP) etching method. As the etching gas,chlorine-based gas represented by Cl₂, BCl₃, SiCl₄, CCl₄ or the like,fluorine-based gas represented by CF₄, SF₆, NF₃, or O₂. Although thereare no specific limitations to the etching gases used, it is suitablehere to use BCl₃ and Cl₂ and O₂. For the etching, A gas flow rate of theabove gas is set at 65,10/5 sccm while RF (13.56 MHz) electrical powerof 450 W is applied to a coiled electrode at a pressure of 1.2 Pa toproduce plasma. RF (13.56 MHz) electric power of 300 W is supplied tothe substrate side (sample stage) also to apply a substantially negativeself-bias voltage. With this first etching condition, the Ti film andAl—Ti film are etched to have an edge portion of the first conductivefilm tapered.

Thereafter, the first etching condition is changed to a second etchingcondition to perform etching while CF₄, Cl₂ and O₂ are used as etchinggas, a gas flow rate is set at 25/25/10 sccm, and RF (13.56 MHz)electric power of 500 W is applied to the coiled electrode at a pressureof 1 Pa to thereby generate plasma. RF (13.56 MHz) electric power of 20W is applied to the substrate side (specimen stage) to thereby apply asubstantially negative self-bias voltage.

In this first etching treatment, it makes edge portions of the firstconductive film and the second conductive film have a tapered shape todesign the shape of the resist mask appropriately and apply the biasvoltage to the substrate side. This tapered portion has an angle of 15to 45°. In this way, owing to the first etching treatment, first shapedgate electrodes 112 to 115 consisting of the first electrode, the secondelectrode and the third electrode (first electrodes 112 a to 115 a,second electrodes 112 b to 115 b and third electrodes 112 c to 115 c)are thus formed (FIG. 3D and FIG. 4D). A region of the gate insulatingfilm which is not covered by the first shaped gate electrodes 112 to 115is etched away by about 20 to 50 nm to be thinned.

Here, a first doping process is performed to thereby dope into asemiconductor layer an impurity element for giving n-type (referred toas “n-type impurity element” hereinafter). Here, the n-type impurityelement is added by ion dope method in a self-align fashion while themask 111 used for forming the first electrodes is left and keptunchanged and also the first shaped gate electrode as is used as a mask.As the n-type impurity element, an element, such as phosphorus (P),arsenic (As) or the like, which belongs to the XV group in the periodictable is used. Here, phosphorus is used. With such an ion dope method,an n-type impurity region which contains the n-type impurity element ata concentration of 1×10²⁰ to 1×10²¹/cm³ is formed in the first impurityregions 116 to 119, an n-type impurity region (A). At this time, it isconsidered that there is also the possibility that an impurity elementis doped into the semiconductor layer through the tapered portion of thefirst electrode and the gate insulating film to form an impurity regionwhich overlaps the first shaped gate electrode.

Next, a second etching treatment is performed without removal of theresist mask 111. As etching gas, chlorine-based gases represented byCl₂, BCl₃, SiCl₄, CCl₄ or the like, fluorine-based gas represented byCF₄, SF₆, NF₃ or the like, or O₂ may he used appropriately. Note herethat although there are no specific limitations to the etching gasesused, it is suitable here to use BCl₃ and Cl₂ and O₂. A gas flow rate ofthe above gas is set at 65/10/5 sccm while RF (13.56 MHz) electricalpower of 450 W is supplied to the coiled electrode at a pressure of 1.2Pa to generate plasma. RF (13.56 MHz) electric power of 300 W issupplied to the substrate side (sample stage) also to thereby apply asubstantially negative self-bias voltage.

Subsequently, CF₄ and Cl₂ plus O₂ are used as etching gas to performetching for about 30 seconds while a gas flow rate is set at 25/25/10sccm and RF (13.56 MHz) electric power of 500 W is applied to the coiledelectrode at a pressure of 1 Pa to thereby produce a plasma. Also, RF(13.56 MHz) electric power of 20 W is applied to the substrate side(sample stage) to thereby apply a substantially negative self-biasvoltage.

In this way, the first electrodes 112 a to 115 a, the second electrodes112 b to 115 b, and the third electrodes 112 c to 115 c, are etched tothus form second shaped gate electrodes 120 to 123 (fourth electrodes120 a to 123 a, fifth electrodes 120 b to 123 b, and sixth electrodes120 c to 123 c) which consist of the fourth electrode, fifth electrodeand sixth electrode.

Then, a second doping process is performed to add an n-type impurityelement to the semiconductor layers 103 to 106. In this process, thesecond shaped gate electrodes 120 to 123 are used as a mask to formn-type impurity regions 124 to 127 containing therein the n-typeimpurity element at a concentration of 1×10¹⁷ to 1×10²⁰/cm³, an n-typeimpurity region (B). At this time, it is considered that there is alsothe possibility that an impurity element is added to the semiconductorlayer through tapered portion of the fourth electrode and the gateinsulating film to form an impurity region overlapping the second shapedgate electrode.

Subsequently, a region that will later become an n-channel type TFT iscovered with masks 128 and 129, and then a third doping process isperformed for doping into the semiconductor layers 104 and 106 animpurity element that gives p-type (referred to hereinafter as p-typeimpurity element). In the third doping process, the second shapedconductive layers are also used as a mask to dope the p-type impurityelement in a self-alignment fashion. Then, p-type impurity regions 130and 131 are formed, which contain the p-type impurity element at aconcentration of 2×10²⁰ to 3×10²¹/cm³.

Here, looking at the p-type impurity regions 130 and 131 in detail, itcan be seen that there are regions 130 a and 131 a containing n-typeimpurity element at a concentration of 1×10²⁰ to 1×10²¹/cm³ and regions130 b and 131 b containing n-type impurity element at a concentration of1×10¹⁷ to 1×10²⁰/cm³. However, in these impurity regions, theconcentration of p-type impurity element is 1.5 to 3 times greater thanthe concentration of n-type impurity element, no problems occur in thecase of functioning as the source region or drain region of a p-channeltype TFT.

Note that the impurity region 131 is formed in a semiconductor layerwhich forms a holding capacitor in the pixel portion.

Up to the steps stated above, an impurity region with a conductivitytype of n-type or p-type is formed in each of the semiconductor layers.Additionally, the second shaped electrode 123 becomes one electrode forthe holding capacitor in the pixel portion.

Next, a first interlayer insulating film 132 a is formed (FIG. 5C andFIG. 6C). This first interlayer insulating film 132 is formed of aninsulating film which contains silicon and hydrogen by plasma CVD methodor sputtering method to a thickness of 100 to 200 nm. One preferredexample is a silicon oxynitride film formed by plasma CVD method to havea film thickness of 150 nm. Needless to say, the first interlayerinsulating film 132 a should not be limited to the silicon oxynitridefilm, and other silicon-containing insulating films may be used in theform of a single layer or a laminated structure.

Thereafter, a process for activating the impurity elements added torespective semiconductor layers is performed with the second harmonicwave (532 nm) of YAG laser to irradiate onto the semiconductor layers.Similarly, An RTA method with a lamp light source may be used, and thesemiconductor layers are heated up by radiating the lamp light sourcefrom the both surfaces of the substrate or from the substrate side.

Thereafter, an insulating film 132 b of silicon nitride is formed byplasma CVD method to have a thickness of 50 to 100 nm, and thermaltreatment at 410° C. is performed with a clean oven to performhydrogenation of the semiconductor layers with hydrogen as released outof the silicon nitride film.

Next, on the first interlayer insulating film 132 b, a second interlayerinsulating film 133 of an organic insulating material is formed. Next,contact holes to reach respective impurity regions are formed.Thereafter, a material such as Al, Ti, Mo, W or the like is used to formwirings and a pixel electrode. For instance, a laminate film of a Tifilm with a film thickness of 50 to 250 nm and an alloy film with a filmthickness of 300 to 500 nm (alloy film of Al and Ti) is used. In thisway, wirings 134 to 140 and a pixel electrode 141 are thus formed (FIG.5D, FIG. 6D).

In the way, it is possible to form on the same substrate a drive circuit201 which has a p-channel type TFT 203 and an n-channel type TFT 204 anda pixel portion 202 which has an n-channel type TFT 205 and a holdingcapacitor 206. This substrate is called the active-matrix substrate forthe purpose of convenience in explanation. Note that the TFT of thepixel portion 202 may alternatively be a p-channel type TFT.

The p-channel type TFT 203 of the drive circuit 201 has a channelformation region 210, the p-type impurity region 130 b and the p-typeimpurity region 130 a which serves as either a source region or drainregion. The n-channel type TFT 204 has a channel formation region 211,the n-type impurity region (B) 125 that becomes an LDD region, and then-type impurity region (A) 117 that functions as either a source regionor drain region. Using these n-channel type TFT and p-channel type TFTmakes it possible to form a shift register circuit, buffer circuit,level shifter circuit, latch circuit and so forth. In particular, thestructure of n-channel type TFT 204 is suitable for the buffer circuitwith a high driving voltage for the purpose of preventing degradationdue to hot carrier effects.

The pixel TFT 205 of the pixel portion 202 a channel formation region212, the n-type impurity region (B) 126 that becomes an LDD region, andthe n-type impurity region (A) 119 that functions as a source region ordrain region. Additionally, in the semiconductor layer that functions asone electrode of the holding capacitor 206, the region to which p-typeimpurity 131 is doped is formed. The holding capacitor 206 is formed ofthe second shaped electrode 123 and semiconductor layer 106 with aninsulating film (the same film as the gate insulating film) being as itsdielectric.

The present invention is capable of optimizing the structure of the TFTwhich forms each circuit in accordance with circuit specificationsrequired by the pixel portion and driver circuit to thereby improve theoperating performance and reliability of a semiconductor device. Morepractically, the n-channel type TFT is designed so as to have the LDDstructure in accordance with the circuit specifications required. Aspreviously stated, the n-channel type TFT of driver circuit is designedto have an LDD structure partially overlapping with the gate electrodeand to be a structure for preventing TFT degradation mainly due to hotcarrier effects. Further, the n-channel type TFT of the pixel portionhave an LDD structure which does not overlap the gate electrode to havea structure for mainly reducing off current. The present inventionprovides a technique for fabricating on the same substrate a p-channeltype TFT in addition to n-channel type TFTs which have differentstructures with six photo-masks. In addition, forming the pixelelectrode of a transparent conductive film makes it possible to form atransmission type display device although a single extra photomask isadditionally required.

Embodiment 2

As an example of a combination of conductive films for the gateelectrode, a semiconductor film represented by a polycrystalline siliconfilm with an impurity element such as phosphorus or else doped may beused as the first conductive film in place of the material as shown inEmbodiment 1. Further, there are another combinations wherein the firstconductive film is formed of a tungsten (W) film or molybdenum (Mo) filmwhile the second conductive is formed of an alloy film of an aluminum(Al) film and a copper (Cu) film and the third conductive film is formedof a titanium (Ti) film, wherein the first conductive film is formed ofa tantalum nitride (TaN) film while the second conductive is formed ofan alloy film of aluminum (Al) and silicon (Si) with the thirdconductive film formed of a titanium (Ti) film, wherein the firstconductive film is formed of a tantalum nitride (TaN) film while thesecond conductive is formed of an aluminum (Al) film, and wherein thefirst conductive film is formed of a titanium nitride (TiN) film whilethe second conductive is formed of an aluminum (Al) film and the thirdconductive film is formed of titanium nitride (TiN).

The above combinations are etched with the same etching gas and the samefloe rate as in Embodiment 1.

Embodiment 2 may be used in combination with the process of Embodiment 1for fabricating the gate electrode.

Embodiment 3

In Embodiment 3, the explanation is given with respect to, in the casewhere a conductive film of Al or containing Al as its main component isused for forming the gate electrode, a method of processing by plasma asurface of the conductive film material to thereby oxidize the surfaceand prevent Al from diffusing into a semiconductor film.

In an oxygen atmosphere or an atmosphere containing oxygen, plasma isperformed to the surface of the conductive of Al or containing Al as itsmain component to oxidize the surface and form an Al_(x)O_(1-x) film.Modifying the quality of the surface of Al through oxidation makes itpossible to prevent aluminum elements from diffusing into thesemiconductor layer.

This Embodiment 3 is used in combination with the process forfabricating the gate electrode Embodiment 1 and 2.

Embodiment 4

In this embodiment 4, a process of forming the semiconductor layer of aTFT is explained with reference to FIG. 7. It should be noted that thecrystallization means in Embodiment 4 is a technique disclosed inEmbodiment 1 of Japanese Patent Laid-Open No. 130652/1995.

First, on a substrate (glass substrate in Embodiment 4) 401, aninsulating base film 402 formed of a silicon oxynitride film with athickness of 200 nm and an amorphous semiconductor film (amorphoussilicon film in Embodiment 4) 403 with a thickness of 200 nm are formed.The insulating base film and amorphous semiconductor film may be formedcontinuously without being opened to the atmospheric air.

Next, with spin coat method, an aqueous solution (nickel acetate watersolution) which contains a 10 ppm by weight of catalytic element (inEmbodiment 4, nickel) conversion applied thereto, thereby to form acatalytic element-containing layer 404 on the entire surface of theamorphous semiconductor film 403. Examples of the usable catalyticelement here include, although not limited to, iron (Fe), nickel (Ni),cobalt (Co), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os),iridium (Ir), platinum (Pt), copper (Cu), and gold (Au) (FIG. 7A).

Further, although the method of adding nickel by spin coat method isused in Embodiment 4, there may be employed a means of using anevaporation method or a sputter method to form a thin-film containing acatalytic element (nickel in the case of this embodiment) on theamorphous semiconductor film.

Next, prior to the process of crystallization, a thermal treatment isperformed at a temperature of 400 to 500° C. for about 1 hour to desorbhydrogen from the film. Thereafter, a thermal treatment at 500 to 650°C. (preferably, 550 to 570° C.) for 4 to 12 hours (preferably, 4 to 6hours) is performed, and in Embodiment 4, the thermal treatment isperformed at 550° C. for 4 hours to a crystalline semiconductor film(crystalline silicon film, in Embodiment 4) 405 (FIG. 7B).

Note here that a laser annealing process (first optical annealing) whichis similar to that of Embodiment 1 may be performed to improve thecrystallinity of the crystalline semiconductor film 405.

Next, a gettering process for removing the nickel used in thecrystallization process from the crystalline silicon film is performed.First, on the surface of the crystalline semiconductor film 405, aninsulating mask film 406 is formed to have a thickness of 150 nm, andopenings 407 are formed by patterning. Then, a process of doping anelement (in Embodiment 4, phosphorus) belonging to the XV group of theperiodic table to an exposed portion of the crystalline semiconductorfilm, and gettering regions 408 containing phosphorus at a concentrationof 1×10¹⁹ to 1×10²⁰ atoms, cm³ are formed (FIG. 7C).

Next, a thermal treatment is performed in a nitrogen atmosphere at 450to 650° C. (preferably, 500 to 550° C.) for 4 to 24 hours (preferably, 6to 12 hours). With this thermal treatment, the nickel within thecrystalline semiconductor film moves in the directions of arrows andthen is captured in the gettering region 408 by gettering actions of thephosphorus. To be brief, since the nickel is removed from within thecrystalline semiconductor film, it is possible to reduce theconcentration of the nickel contained in the crystalline semiconductorfilm 409 to a level less than or equal to 1×10¹⁷ atoms/cm³, andpreferably 1×10¹⁶ atoms/cm³ or less (FIG. 7D).

The crystalline semiconductor film 409 thus formed in the way statedabove is a crystalline semiconductor film with noticeably excellentcrystallinity since a catalytic element (here, nickel) for acceleratingcrystallization. Additionally, the catalytic element is removed away bygettering action of the phosphorus after the crystallization, theconcentration of the catalytic element within the crystallinesemiconductor film 409 (except gettering regions) is less than or equalto 1×10¹⁷ atoms/cm³, and preferably 1×10¹⁶ atoms/cm³ or below.

Note here that Embodiment 4 is characterize in that the getteringregions (the regions containing an impurity element belonging to the XVgroup of the periodic table at a high concentration) are formed incertain regions which are not become active layers after forming thecrystalline semiconductor film using a catalytic element, and that thecatalytic element used for the crystallization is subjected to getteringby thermal treatment.

The structure of Embodiment 4 is usable in combination with thecrystallization process of the semiconductor film shown in Embodiment 1and the manufacturing process of the gate electrode indicated inEmbodiment 2.

Embodiment 5

In Embodiment 5, a process of forming a semiconductor layer of a TFTwith reference to FIG. 8. Practically, the technique disclosed inPublished Japanese Patent Laid-Open No. 247735/1996 (U.S. patentapplication Ser. No. 09/034,041) is employed.

First, on a substrate (glass substrate in Embodiment 5) 501, aninsulating base film 502 formed of a silicon oxynitride film with athickness of 200 nm and an amorphous semiconductor film (amorphoussilicon film in Embodiment 5) 503 with a thickness of 200 nm are formed.This process is alterable in such a way that the dielectric undercoatfilm and amorphous semiconductor film are formed continuously withoutbeing opened to the atmospheric air.

Next, an insulating mask film 504 of a silicon oxide film is formed tohave a thickness of 200 nm, and an opening 505 is formed.

Next, an aqueous solution (nickel acetate water solution) containing 100ppm by weight of catalytic element (nickel in Embodiment 5) is appliedby spin coating method to form a catalytic element-containing layer 506.At this time, the catalytic element-containing layer 506 is selectivelybrought into contact with the amorphous semiconductor film 503 in aregion of the opening 505. Examples of the usable catalytic element hereinclude, although not limited to, iron (Fe), nickel (Ni), cobalt (Co),ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir),platinum (Pt), copper (Cu), and gold (Au) (FIG. 8A).

Although the method of adding nickel by spin coat method is used inEmbodiment 5, there may be employed a means of an evaporation method ora sputter method to form on the amorphous semiconductor film a thin-filmincluding a catalytic element (nickel in the case of Embodiment 5).

Next, prior to execution of the crystallization process, a thermaltreatment is performed at 400 to 500° C. for about 1 hour to desorbhydrogen from within the film, and a thermal treatment at 500 to 650° C.(preferably, 550 to 600° C.) for 6 to 16 hours (preferably, 8 to 14hours), at 570° C. for 14 hours in Embodiment 5. As a result,crystallization progresses in a direction (the direction as indicated byarrow) which is approximately parallel to the substrate with the opening505 as a start point, thereby to form a crystalline semiconductor film(in Embodiment 5, crystalline silicon film) 507 with alignment of macrocrystal growth directions (FIG. 8B).

Next, a gettering process is performed for removing the nickel used inthe crystallization process from the polycrystalline silicon film. InEmbodiment 5, a process of doping an element (phosphorus, in Embodiment5) which belongs to the XV group of the periodic table with thepreviously formed dielectric mask film 50 used as a mask, thus to form agettering region 508 which contains therein phosphorus with aconcentration of 1×10¹⁹ to 1×10²⁰ atoms/cm³ in the crystallinesemiconductor film thus exposed at the opening 505 (FIG. 8C).

Next, a thermal treatment in a nitrogen atmosphere at 450 to 650° C.(preferably, 500 to 550° C.) for 4 to 24 hours (preferably 6 to 12hours) is conducted. With the thermal treatment, the nickel within thecrystalline semiconductor film moves in a direction of arrow, and thenis captured in the gettering region 508 by gettering actions of thephosphorus. To be brief, the nickel is removed from the crystallinesemiconductor film: thus, and it is possible to reduce the concentrationof the nickel contained in the crystalline semiconductor film 509 to theintended level that is less than or equal to 1×10¹⁷ atoms/cm³, andpreferably 1×10¹⁶ atoms/cm³ or less (FIG. 8D).

The crystalline semiconductor film 509 thus formed in the way statedabove is formed of a crystalline semiconductor film which is noticeablyexcellent in crystallinity for selectively doping a catalytic element(here, nickel) for accelerating crystallization. More practically, thecrystalline semiconductor film has a crystal structure in whichstem-shaped or pole-like crystals are aligned while with specificdirectional properties. Additionally, the catalytic element is removedby gettering actions of the phosphorus after the crystallization, andthe concentration of the catalytic element remaining within thecrystalline semiconductor film 509 is expected to stay at a level lessthan or equal to 1×10¹⁷ atoms/cm³, and preferably 1×10¹⁶ atoms/cm³ orless.

Note here that Embodiment 5 is characterized in that the getteringregion (the region containing at high concentration an impurity elementwhich belongs to the XV group of the periodic table) are formed in acertain region which is not used as an active layer after forming thecrystalline semiconductor film crystallized using a catalytic element,and that the catalytic element used for the crystallization is subjectedto gettering by thermal treatment.

The construction of Embodiment 5 is employable in combination with themethod of fabricating semiconductor device which is indicated inEmbodiments 1 and 2.

Embodiment 6

A method of forming a semiconductor layer of a TFT is described inEmbodiment 6 with reference to FIGS. 9A to 9D, in which a metal elementwith a catalytic function is added over the entire surface of anamorphous semiconductor film for crystallizing, a semiconductor filmcontaining a rare gas element (Ar in Embodiment 6) is formed to be usedas a gettering site.

In FIG. 9A. Although there is no particular limit to the material of asubstrate 600, barium borosilicate glass, aluminoborosilicate glass,quartz, or the like can be preferably used. On the surface of thesubstrate 600, an inorganic insulating film is formed into a thicknessof 10 to 200 nm as a base insulating film 601. An example of apreferable base insulating film includes a silicon oxynitride filmformed by plasma CVD. A first silicon oxynitride film with a thicknessof 50 nm made of SiH₄, NH₃, and N₂O and a second silicon oxynitride filmhaving a thickness of 100 nm made of SiH₄ and N₂O are used as the baseinsulating film 601. The base insulating film 601 is provided for thepurpose of preventing alkali metal contained in a glass substrate fromdiffusing into a semiconductor film formed on the base insulating film601. In the case of using quartz as a substrate, the base insulatingfilm 601 may be omitted.

An amorphous semiconductor film 602 formed on the base insulating film601 is a semiconductor material containing silicon as a main component.Typically, an amorphous silicon film, an amorphous silicon germaniumfilm, or the like is formed into a thickness of 10 to 100 nm by plasmaCVD, low-pressure CVD, or sputtering. In order to obtain a satisfactorycrystal, the concentration of impurities such as oxygen and nitrogencontained in the amorphous semiconductor film 602 may he lowered to5×10¹⁸/cm³ or less. These impurities hinder crystallization of anamorphous semiconductor, and increase the density of a trapping centerand a recombination center even after crystallization. Therefore, it isdesirable to use a CVD apparatus designed for ultra-high vacuum, whichis subjected to mirror-surface treatment (electrical field polishingtreatment) in a reaction chamber and is equipped with an oil-free vacuumexhaust system, as well as to use a material gas with a high purity.

Thereafter, a metal element with a catalytic function of promotingcrystallization is added to the surface of the amorphous semiconductorfilm 602 (FIG. 9B). Examples of such a catalytic element include iron(Fe), nickel (Ni), cobalt (Co), ruthenium (Ru), rhodium (Rh), palladium(Pd), osmium (Os), iridium (Ir), platinum (Pt), copper (Cu), gold (Au),and the like, and one kind or a plurality of kinds selected from theseelements can be used. Typically, nickel is used. The amorphoussemiconductor film 602 is coated with a nickel acetate solutioncontaining 1 to 100 ppm by weight of nickel with a spinner, thereby toform a catalytic element containing layer 603. In this case, in order toenhance compatibility of the solution, the amorphous semiconductor film602 is subjected to a surface treatment as follows: a very thin oxidefilm is formed with an ozone-containing aqueous solution, the oxide filmis etched with a mixed solution of fluoric acid and hydrogen peroxide toobtain a clean surface, and the resultant surface is treated with anozone-containing aqueous solution again to form a very thin oxide film.Since the surface of a semiconductor film such as silicon ishydrophobic, the surface of the amorphous semiconductor film 602 can beuniformly coated with a nickel acetate solution by forming art oxidefilm as described above.

Needless to say, there is no particular limit to the method of formingthe catalytic element containing layer 603, and the catalytic elementcontaining layer 603 may be formed by sputtering, evaporation, plasmatreatment, or the like. Furthermore, the catalytic element containinglayer 603 may be formed on the base insulating film 601 before formingthe amorphous semiconductor film 602.

While the amorphous semiconductor film 602 is in contact with thecatalytic element containing layer 603, a heat treatment forcrystallization is conducted. As a method for the heat treatment, afurnace annealing using an electrothermal furnace, or rapid thermalannealing (hereinafter, referred to as “RTA”) using a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, a high-pressure mercury lamp, or the like are adopted. Inview of productivity, RTA is preferably adopted.

In the case of conducting RTA, a lamp light source for heating isswitched on for 1 to 60 seconds, preferably 30 to 60 seconds andlighting is repeated 1 to 10 times, preferably 2 to 6 times. Althoughthe light-emitting intensity of a lamp light source is arbitrarily set:the intensity is set in order that the semiconductor film is rapidlyheated up to about 600° C. to 1000° C. preferably about 650° C. to about750° C. Even at such a high temperature, only the semiconductor film israpidly heated, and the substrate 600 itself is not strained to bedeformed. Thus, the amorphous semiconductor film is crystallized toobtain a crystalline semiconductor film 604 as shown in FIG. 9C. Suchcrystallization can be achieved by providing the catalytic elementcontaining layer 603.

In the case of using furnace annealing as the other method, prior to theheat treatment for crystallization, the amorphous semiconductor film 602is subjected to a heat treatment at 500° C. for about one hour, wherebyhydrogen contained in the amorphous semiconductor film 602 is released.Then, a heat treatment is conducted in a nitrogen atmosphere at 550° C.to 600° C. preferably at 580° C. for four hours, using an electrothermalfurnace, and thus the amorphous semiconductor film 602 is crystallized.Accordingly, the crystalline semiconductor film 604 as shown in FIG. 9Cis formed.

In order to enhance a crystallization ratio (ratio of a crystalcomponent to the entire volume of the film), and repair defectsremaining in the crystal grains, it is also effective to irradiate thecrystalline semiconductor film 604 with laser light as shown in FIG. 9C.As the laser, excimer laser light with a wavelength of 400 nm or less,the second harmonic or the third harmonic of YAG laser is used. In anycase, the crystalline semiconductor film 604 may be subjected to a lasertreatment with an overlap ratio of 90 to 95% by using pulse laser lightwith a repeating frequency of about 10 to 1000 Hz and condensing thelaser light at 100 to 400 mJ/cm² with an optical system.

In a crystalline semiconductor film 605 thus obtained, a catalyticelement (herein, nickel) remains. Although not uniformly distributed inthe film, the catalytic element remains at an average concentration over1×10¹⁹/cm³. Needless to say, even in such a state, it is possible toform various semiconductor devices such as a TFT. However, the catalyticelement is removed by gettering using the following method.

First, as shown in FIG. 9D, a thin layer 606 is formed on the surface ofthe crystalline semiconductor film 605. In the present specification,the thin layer 606 provided on the crystalline semiconductor film 605 isprovided to prevent the first semiconductor film 605 from being etchedwhen a gettering site is removed later. Therefore, the thin layer 606will be referred to as a “barrier layer 606”.

The thickness of the barrier layer 606 is set to he about 1 to 10 nm,and chemical oxide formed by a treatment with ozone water may be used asa barrier layer simply. Furthermore, even by a treatment with an aqueoussolution containing a mixture of sulfuric acid, hydrochloric acid, ornitric acid together with hydrogen peroxide, a chemical oxide film canbe similarly formed. Alternatively, a plasma treatment in an oxidizingatmosphere, or oxidation treatment in which ozone is generated byirradiating UV-light in an oxygen-containing atmosphere or the like maybe conducted. Furthermore, a thin oxide film is formed to obtain aharrier layer by heating at about 200° C. to about 350° C. with a cleanoven. Alternatively, an oxide film with a thickness of about 1 to 5 nmis deposited to obtain a harrier layer by plasma CVD, sputtering,evaporation, or the like. In any case, a film which allows a catalyticelement to move to a gettering site side in the gettering process anddoes not allow an etchant to penetrate into the film (protects thecrystalline semiconductor film 605 from an etchant) in the removingprocess of a gettering site may be used. For example, a chemical oxidefilm formed by a treatment with ozone water, a silicon oxide film(SiO_(x)), or a porous film may be used.

Then, as a gettering site 607, a second semiconductor film (typically,an amorphous silicon film) containing a rare gas element at aconcentration of 1×10²⁰/cm³ or more is formed on the barrier layer 606to have a thickness of 25 to 250 nm by sputtering. Since the getteringsite 607 is removed later, a low-density film is preferably formed so asto obtain a large selection ratio of etching with respect to thecrystalline semiconductor film 605.

In the case of forming the gettering site 607 by sputtering under theconditions that a gas (Ar) flow rate is 50 (sccm), a film formationpower is 3 kW, a substrate temperature is 150° C., and the filmformation pressure is varied from 0.2 to 1.0 Pa, the concentration of arare gas in the gettering site 607 is set 1×10¹⁹/cm³ to 1×10²²/cm³,preferably 1×10²⁰/cm³ to 1×10²¹/cm³, more preferably 5×10²⁰/cm³, andallows a gettering effect.

As being inactive itself in a semiconductor film, a rare gas elementdoes not adversely influence the crystalline semiconductor film 605. Asa rare gas element, one kind or a plurality of kinds selected fromhelium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe) areused. The present invention is characterized in that these rare gaselements are used as ion sources for forming a gettering site, and asemiconductor film containing these elements is formed to obtain agettering site.

In order to surely achieve gettering, it is required to conduct a heattreatment thereafter. The heat treatment is conducted by furnaceannealing or RTA. In the case of adopting furnace annealing, a heattreatment is conducted at 450° C. to 600° C. for 0.5 to 12 hours in anitrogen atmosphere. In the case of RTA, a lamp light source for heatingis switched on for 1 to 60 seconds, preferably 30 to 60 seconds andlighting is repeated 1 to 10 times, preferably 2 to 6 times. Althoughthe light-emitting intensity of a lamp light source is arbitrarilydetermined, the intensity is set in order that semiconductor film israpidly heated to about 600° C. to about 1000° C. preferably about 700°C. to about 750° C.

During gettering, a catalytic element in a region to be gettered (trapsite) is released by heat energy, and moved to a gettering site bydiffusion. Thus, gettering depends upon a treatment temperature, andproceeds in a shorter period of time at a higher temperature. Accordingto the present invention, the distance where a catalytic element movesduring gettering corresponds to approximately the thickness of asemiconductor film, as shown by an arrow in FIG. 9D, whereby getteringcan be completed in a relatively short period of time.

Even in the above-mentioned heat treatment, the semiconductor film 607containing a rare gas element at a concentration of 1×10¹⁹/cm³ to1×10²¹/cm³, preferably 1×10²⁰/cm³ to 1×10²¹/cm³, more preferably5×10²⁰/cm³ is not crystallized. The reason for this is considered asfollows: a rare gas element remains in a semiconductor film withoutbeing released again even in the above-mentioned range of a heattreatment temperature, thereby to inhibit crystallization of thesemiconductor film.

After completing gettering, the amorphous semiconductor 607 is removedby selectively etching. The etching can be conducted by dry etching withClF₃, without using plasma, or wet etching with an alkaline solutionsuch as hydrazine and an aqueous solution containing tetraethyl ammoniumhydroxide ((CH₃)₄NOH). At this time, the barrier layer 606 functions asan etching stopper. The barrier layer 606 may be removed with fluoricacid thereafter.

Accordingly, a crystalline semiconductor film 608 with a concentrationof a catalytic element reduced to 1×10¹⁷/cm³ or less can be obtained asshown in FIG. 9E.

Embodiment 6 can be used by combining gettering process of the catalystelement which is executed after the formation of the crystallinesemiconductor film in the method of forming the semiconductor device inEmbodiments 1 or 2.

Embodiment 7

In Embodiment 7, the manufacturing process of a semiconductor layer of aTFT is described using FIG. 10, in which a metal element with acatalytic function is added to the entire surface of an amorphoussemiconductor film for crystallizing while gettering is simultaneouslyconducted.

First, as shown in FIG. 10A, a catalytic element containing layer 702 isformed on a base insulating film 701. At this time, an aqueous solutioncontaining a catalytic element or alcohol solution may be used with aspinner. Alternatively, the catalytic element containing layer 702 maybe formed by sputtering, evaporation, a plasma treatment, or the like.

Thereafter, an amorphous semiconductor film 703 is formed into athickness of 10 to 100 nm by plasma CVD, low-pressure CVD, orsputtering, as shown in FIG. 10B. Furthermore, a barrier layer 704 isformed. The method of forming these films is the same as that inEmbodiment 6.

Then, a semiconductor film 705 containing a rare gas element in aconcentration of 1×10¹⁹/cm³ to 1×10²²/cm³, preferably 1×10²⁰ to1×10²¹/cm³, more preferably 5×10²⁰/cm³ is formed into a thickness of 25to 250 nm by plasma CVD. Typically, an amorphous silicon film isselected. Since the semiconductor film 705 is removed later, alow-density film is desirably formed.

Next, a heat treatment is conducted. As a method of the heat treatment,a furnace annealing using an electrothermal furnace, or RTA using ahalogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp,a high-pressure sodium lamp, a high-pressure mercury lamp, or the likeare conducted.

In the above-mentioned heat treatment, a catalytic element permeatesinto the semiconductor film 703 with an amorphous structure, anddiffuses toward the semiconductor film 705 (in a direction representedby an arrow in FIG. 10B) with crystallizing the semiconductor film 703.Because of this, crystallization and gettering are simultaneouslyconducted by one heat treatment.

Thereafter, the semiconductor film 705 is removed by selectivelyetching. The etching can be conducted by dry etching with ClF₃, withoutusing plasma, or wet etching with an alkaline solution such as hydrazineand an aqueous solution containing tetraethyl ammonium hydroxide((CH₃)₄NOH). At this time, the barrier layer 704 functions as an etchingstopper. The barrier layer 704 may be removed with fluoric acidthereafter.

Thus, a semiconductor film (first semiconductor film) 706 with a crystalstructure and a concentration of a catalytic element reduced to1×10¹⁷/cm³ or less can be obtained as shown in FIG. 10D. In order toenhance the crystallinity of the crystalline semiconductor film 706, thesemiconductor film 706 may he irradiated with laser light.

Thus, a crystalline semiconductor film 707 with a concentration of acatalytic element reduced to 1×10¹⁷/cm³ or less can be obtained as shownin FIG. 10D. Embodiment 7 can be used by combining with themanufacturing process of the crystalline semiconductor film in themethod of manufacturing the semiconductor device of Embodiment 1.

Embodiment 8

In Embodiment 8, the steps to fabricate a liquid crystal module madefrom the active matrix substrate obtained in Embodiment 1 are describedbelow.

An orientation film is formed on the active matrix substrate, followedby conducting rubbing processing. In Embodiment 8, before theorientation film is formed, an organic resin film such as an acrylicresin film is patterned to form column-shaped spacers at desiredpositions for the purpose of keeping a substrate gap. In place of thecolumn-shaped spacers, spherical spacers may be dispersed over theentire surface of the substrate.

Then, an opposite substrate is prepared. A color filter in which acoloring layer and a light-shielding layer are provided to correspond toeach pixel is formed over the opposite substrate. Further, alight-shielding layer is formed over the driving circuit. In addition, aleveling film covering the color filter and the light-shielding layer isformed. Then, an opposite electrode of a transparent conductive film isformed on the leveling film at least in the pixel portion, and anorientation film is formed over the entire surface of the oppositesubstrate, followed by conducting rubbing.

The active matrix substrate with the pixel portion and the drivingcircuit formed thereon is attached to the opposite substrate with asealant. Filler is mixed in the sealant. Two substrates are attached toeach other while a uniform gap is kept therebetween with the filler andthe column-shaped spacers. Thereafter, a liquid crystal material isinjected between the substrates before completely sealing with asealant. As the liquid crystal material, a known liquid crystal materialmay he used. Then, with described above, the liquid crystal module iscompleted. Further, if necessary, the active matrix substrate or theopposite substrate are divided into the required shape. Furthermore, apolarizing plate etc. is attached by using a known method. Then, a FPCis attached by using a known method.

The structure of the liquid crystal module obtained in such manner isdescribed with reference of a top view of FIG. 11A.

As shown in a top view of FIG. 11, an active matrix substrate isattached to an opposite substrate 300 with a sealant 307. The activematrix substrate is provided with a pixel portion, driving circuits, anexternal input terminal 309 to which an FPC (Flexible Printed Circuit)311 is attached, and a wiring 310 connecting the external input terminalto an input portion of each circuit. The opposite substrate 300 isprovided with a color filter and the like.

A light-shielding layer 303 a is provided at the opposite substrate sideso as to be overlapped with a gate side driving circuit 301 a, and alight shielding layer 303 b is provided at the opposite substrate sideso as to be overlapped with a source side driving circuit 301 b. A colorfilter 302 disposed over the pixel portion 305 at the opposite substrateside is provided to have a light shielding layer and a coloring layer ofeach color (red (R), green (G), and blue (B)) which is corresponding toeach pixel. Actually, a color display is conducted with three colors ofthe coloring layer of red (R), the coloring layer of green (G), and thecoloring layer of blue (B). The coloring layers of the respective colorsare arranged arbitrarily.

Herein, in order to conduct a color display, the color filter 302 isprovided on the opposite substrate. However, the present invention isnot limited thereto. A color filter may be formed over an active matrixsubstrate when the active matrix substrate is manufactured.

In the color filter, a light shielding layer is provided in a regionbetween the adjacent pixels, whereby light is blocked in the regionexcept a display region. Furthermore, the light-shielding layers 303 aand 303 b are provided so as to cover the driving circuits. However,covers is placed over the regions of the driving circuits when theliquid crystal display apparatus is incorporated into electronicequipment as a display portion. Therefore, the regions of the drivingcircuits may not be covered with the light-shielding layers.Furthermore, a light-shielding layer may be formed on an active matrixsubstrate when the active matrix substrate is manufactured.

Furthermore, the following may also be possible. Instead of providingthe above-mentioned light-shielding layers, a plurality of stackedcoloring layers constituting a color filter are appropriately disposedbetween the opposite substrate and the opposite electrode, whereby lightis blocked in the region (gap between the respective pixel electrodes)except a display region and driving circuits.

Furthermore, an FPC 311 composed of a base film and a wiring is attachedto the external input terminal with anisotropic conductive resin.Furthermore, the mechanical strength of the device is enhanced with areinforcing plate. The state taken along the line of A-A′ of externalinput terminal portion is shown in FIG. 11B.

As mentioned above, even if a large-scale display device is formed, theproblem such as a signal delay is never caused by raising the wringresistance. The reason is that the following films are laminated inorder to form the gate electrode as the present invention; Al, a lowresistance conductive film or the conductive film containing Al mainly,W or the conductive film containing W mainly, Ti or the conductive filmcontaining Ti mainly. Thus, the liquid crystal module formed to containthe gate electrode of the present invention can be used as displayportions of various electronic equipments.

Embodiment 9

The CMOS circuit and the pixel portion formed by implementing theinvention can be used in active matrix type liquid crystal displaydevice (liquid crystal display device). That is, the present inventioncan be implemented in all of electronic apparatuses integrated with theelectro-optical devices at display portions thereof.

As such electronic apparatus, there are pointed out a video camera, adigital camera, a projector (rear type or front type), a head mountdisplay (goggle type display), a personal computer, a portableinformation terminal (mobile computer, portable telephone or electronicbook) and the like. Examples of these are shown in FIGS. 12A through12F, FIGS. 13A through 13D and FIGS. 14A through 14C.

FIG. 12A shows a personal computer including a main body 2001, an imageinput portion 2002, a display portion 2003 and a keyboard 2004.

FIG. 12B shows a video camera including a main body 2101, a displayportion 2102, a voice input portion 2103, operation switches 2104, abattery 2105 and an image receiving portion 2106.

FIG. 12C shows a mobile computer including a main body 2201, a cameraportion 2202, an image receiving portion 2203, an operation switch 2204and a display portion 2205.

FIG. 12D shows a goggle type display including a main body 2301, adisplay portion 2302 and an arm portion 2303.

FIG. 12E shows a player using a record medium recorded with programs(hereinafter, referred to as record medium) including a main body 2401,a display portion 2402, a speaker portion 2403, a record medium 2404 andan operation switch 2405. The player uses DVD (Digital Versatile Disc)or CD as the record medium and can enjoy music, enjoy movie and carryout game or Internet.

FIG. 12F shows a digital camera including a main body 2501, a displayportion 2502, an eye contact portion 2503, operation switches 2504 andan image receiving portion (not illustrated).

FIG. 13A shows a front type projector including a projection apparatus2601 and a screen 2602.

FIG. 13B shows a rear type projector including a main body 2701, aprojection apparatus 2702, a mirror 2703 and a screen 2704.

Further, FIG. 13C is a view showing an example of a structure of theprojection apparatus 2601 and 2702 in FIG. 13A and FIG. 13B,respectively. The projection apparatus 2601 or 2702 is constituted by alight source optical system 2801, mirrors 2802, and 2804 through 2806, adichroic mirror 2803, a prism 2807, a liquid crystal display apparatus2808, a phase difference plate 2809 and a projection optical system2810. The projection optical system 2810 is constituted by an opticalsystem including a projection lens. Although the embodiment shows anexample of three plates type, the embodiment is not particularly limitedthereto but may be of, for example, a single plate type. Further, aperson of executing the embodiment may pertinently provide an opticalsystem such as an optical lens, a film having a polarization function, afilm for adjusting a phase difference or an IR film in an optical pathshown by arrow marks in FIG. 13C.

Further, FIG. 13D is a view showing an example of a structure of thelight source optical system 2801 in FIG. 13C. According to theembodiment, the light source optical system 2801 is constituted by areflector 2811, a light source 2812, lens arrays 2813 and 2814, apolarization conversion element 2815 and a focusing lens 2816. Further,the light source optical system shown in FIG. 13D is only an example andthe embodiment is not particularly limited thereto. For example, aperson of executing the embodiment may pertinently provide an opticalsystem such as an optical lens, a film having a polarization function, afilm for adjusting a phase difference or an IR film in the light sourceoptical system.

However, according to the projectors shown in FIGS. 13A, 13B and 13C,there is shown a case of using a transmission type electronic apparatusand an example of applying a reflection type electronic apparatus is notillustrated.

FIG. 14A shows a mobile telephone including a display panel 3001, anoperation panel 3002. The display panel 3001 and the operation panel3002 are connected to each other in the connecting portion 3003. In theconnecting panel 3003, the angle θ of a face which is provided thedisplay portion 3004 of the display panel 3001 and a face which isprovided the operation key 3006 of the operation panel 3002 can bechanged arbitrary. Further, a voice output portion 3005, an operationkey 3006, a power source switch 3007 and a sound input portion 3008 arealso included.

FIG. 14B shows a portable book (electronic book) including a main body3101, display portions 3102 and 3103, a record medium 3104, an operationswitch 3105 and an antenna 3106.

FIG. 14C shows a display including a main body 3201, a support base 3202and a display portion 3203. The display according to the invention isadvantageous particularly in the case of large screen formation and isadvantageous in the display having a diagonal length of 10 inch or more(particularly, 30 inch or more).

As has been described, the range of applying the invention is extremelywide and is applicable to electronic apparatus of all the fields.Further, the electronic apparatus of this embodiment can be realized byusing any constitution comprising any combinations of Embodiments 1 to8.

With the disclosed technique for fabricating wirings by using thematerial with low resistance, it is possible to realize excellentdisplay without problems such as signal delay and the like in displayscreen size in a semiconductor device represented by an active matrixtype liquid crystal display device since the wiring resistance issuppressed even when the area of a pixel portion increases to result inan increase in display screen size.

As it is possible to greatly reduce the resistance of a source wiring inthe pixel portion, the present invention is adaptable as a technique forfabricating large display liquid crystal display devices with diagonal40-inch or diagonal 50-inch screen sizes.

1. A semiconductor device comprising: a pixel portion over a substrate;and an external input terminal portion over the substrate, wherein thepixel portion comprises: a semiconductor layer; a gate electrode; and agate insulating film between the semiconductor layer and the gateelectrode, wherein the gate electrode comprises a laminate of a firstlayer, a second layer and a third layer, and the first layer is incontact with the gate insulating film, and wherein the external inputterminal portion has a first portion and a second portion, the firstportion including a conductive film comprising the first layer, thesecond layer and the third layer, and the second portion including aninterlayer insulating film, wherein the conductive film is covered witha conductive layer, and wherein the first portion and the second portionare arranged alternately.
 2. The semiconductor device as claimed inclaim 1, wherein the semiconductor layer comprises a channel formationregion, a LDD region in contact with the channel formation region, and asource region or a drain region in contact with the LDD region.
 3. Thesemiconductor device as claimed in claim 1, wherein the first layer isformed of a conductive film comprising tungsten or a material includingtungsten as its main component, the second layer is formed of aconductive film comprising copper or a material including copper as itsmain component, and the third layer is formed of a conductive filmcomprising titanium or a material including titanium as its maincomponent.
 4. The semiconductor device as claimed in claim 1, whereinthe gate electrode has a taper shape.
 5. The semiconductor device asclaimed in claim 1, wherein the semiconductor layer is located betweenthe substrate and the gate electrode.
 6. The semiconductor device asclaimed in claim 1, wherein the gate electrode is located between thesemiconductor layer and the interlayer insulating film.
 7. Thesemiconductor device as claimed in claim 1, wherein the semiconductordevice is selected from the group consisting of a computer, a videocamera, a digital camera, a mobile telephone, and a projector.